This invention relates to acrylic coating compositions and, in particular, to a high gloss acrylic coating composition cured with melamine that demonstrates improved impact resistance. More particularly, this invention relates to the incorporation of polytrimethylene carbonate diols and triols into acrylic coatings cured with melamine to obtain high gloss acrylic coatings with improved impact resistance, with no significant loss of other properties.
Thermoset or cured coating compositions are widely used in coatings operations on a variety of substrates, including plastic, metal, wood, primed metals, or previously coated or painted metals. One type of thermosetting coating is an acrylic coating composition. In automotive applications, in particular, acrylic coatings provide durable finishes. Acrylic coating compositions are well known and have been widely used to finish automobiles and trucks.
Automotive coatings include primers and topcoats, which may be single layer topcoats or two layer basecoat/clearcoat topcoat systems. The primer may be applied either as a first coating layer or over another layer, for example over an electrocoat primer layer. The topcoats are then usually applied as a protective coat over the primer layer.
In order to make the coating more chip resistant, one recognized solution is to cover all or parts of the finished surface of the automobile with a protective coating, however the acrylic enamel, acrylic lacquer, or nitrocellulose lacquer typically used on vehicles produce coatings which are difficult to overcoat with protective materials due to problems with adhesion, yellowing, etc. A useful protective coating composition should first and foremost be chip- and abrasion-resistant, have good adhesion to the painted surface, be clear, smooth (i.e., without surface roughness) and indistinguishable over the painted surface when applied to the areas being protected.
There are a number of considerations regarding the use of thermosetting coating compositions. One consideration involves the curing conditions needed to achieve sufficient crosslinking of the film, with higher curing temperatures and longer times at the curing temperature generally increasing the manufacturing costs of the coated article. Another concern in some cases is the generation of undesirable by-products of the curing reaction. For example, blocked curing agents may release the blocking agents as volatile organic compounds that are emissions regulated by various government regulations. It is also important that the crosslinks that are formed by curing thermosetting compositions are suitable for providing long life to the coating under the particular conditions to which the coated article will be exposed.
Several different crosslinking mechanisms may be employed in thermosetting coatings. Polyisocyanate crosslinkers may be reacted with amine or hydroxyl groups on the resin. This curing method provides desirable urea or urethane crosslinked bonds, but may also entail certain drawbacks. In order to prevent premature gelation of the coating composition, the polyisocyanate must either be kept separate from the resin in what is known in the art as a two-package or two-pack coating system, or else the highly reactive isocyanate groups on the curing agent must be blocked (e.g., with an oxime or alcohol). Blocked polyisocyanates, however, require higher temperatures (e.g., 150xc2x0 C., or more) to unblock and begin the curing reaction. The volatile blocking agents released during curing can possibly adversely affect coating properties, as well as increase the volatile organic content for the composition.
Another curing mechanism utilizes a melamine formaldehyde resin curing agent in the coating composition to react with hydroxyl groups on the resin. Where suitable, this curing method provides good cure at relatively low temperatures, for example 250xc2x0 F. or 121xc2x0 C. with a blocked acid catalyst, or even lower with an unblocked acid catalyst, however higher curing temperatures can also be effective.
There are some advantages in curing with melamine where suitable. Melamine can exhibit moisture resistance, cure at lower temperatures, and can be extremely hard, and yet colorless. The moisture resistance feature of melamine-based adhesives, combined with its durability, may provide advantages for exterior applications. Curing temperatures as low as 140xc2x0 F. have been used for melamine adhesives, with a normal range from 240-260xc2x0 F. for 2 to 5 minutes, depending upon the thickness of the composite assembly. Crosslinked melamine-based coatings are colorless, chemically resistant and resilient. They provide a tough and durable finish to items that will be repeatedly exposed to harsh environments.
Few applications experience such demanding and harsh environments over time as automotive paints. In this application melamine can deliver chemical resistance and durability. Melamine resins also provide the long-term buffability vehicle owners desire. Melamine-based coatings also permit coils of metal sheeting to be prepainted, then stamped into the final product, as in the case of appliance and automotive parts and panels.
Another important benefit of high solids melamine-based coatings is that they are low in volatile organic emissions during application and curing.
The use of various modifiers to attempt to improve impact properties of acrylic coatings has been addressed in the art. Polytrimethylene ether glycol (PTMEG) has been suggested as a modifier, however at the expense of optimum UV resistance. The addition of glycol adipates to improve impact resistance has been suggested, but results in the reduction of hydrolytic stability. Impact modifiers previously proposed in the art typically result in the loss of other properties.
The preparation of trimethylene carbonate is known. U.S. Pat. No. 5,212,321 discloses a process for preparing trimethylene carbonate wherein 1,3-propanediol is reacted with diethylcarbonate in the presence of zinc powder, zinc oxide, tin powder, tin halide or organo-tin compound, at an elevated temperature. It is also known in the art to use polytrimethylene carbonate in polyester applications. See, for example, U.S. Pat. Nos. 5,225,129 and 5,849,859.
The preparation of polycarbonate polyols is known in the art. U.S. Pat. No. 4,533,729 discloses a process for preparing amorphous polycarbonate polyols by reacting phosgene, a branched-chain polyhydric alcohol, and a straight chain polyhydric alcohol in the presence of a solvent and in the absence of a catalyst at a temperature of from about 60xc2x0 to 100xc2x0 C. The reaction mixture is then contacted with a catalytic amount of a tertiary amine at reflux temperature for a period of time of at least about 30 minutes. It is suggested the resulting polycarbonate polyol can be used in coating compositions. In JP 64001724 there is disclosed the preparation of a polycarbonate polyol from (di) allyl-, alkyl- or alkylene carbonate and a polyhydroxy compound using a titanium catalyst.
Polycarbonates have been used in acrylic and polyester coatings. U.S. Pat. No. 5,525,670 describes a coating composition based on either acrylic or polyester resins modified with polycarbonates which are cured by either urethane or melamine formaldehyde chemistries. The polycarbonate described preferably has a number average molecular weight above 2000. The polycarbonate of this reference is made from a mixture of straight chain diols, branched chain diols, and polyhydric alcohols and an aliphatic carbonate, where both the branched chain diols and the polyhydric alcohols are present in at least 10 mol %. It is stated in this reference that if less than 10 mol % is present, the material crystallizes (branched chain diol), and inferior curing characteristics (polyhydric alcohols) and inferior water resistance (polyhydric alcohols) are exhibited. Also see U.S. Pat. No. 5,527, 879.
EP 0 712 873 A2 describes an acrylic copolymer which is an acrylic monomer having a hydroxy alkyl carbonate group and an acid group-containing monomer. The composition is said to be crosslinked with melamine to prepare a thermosetting water borne coating composition.
There does not appear to be any reference in the art that discloses or suggests the use of polytrimethylene carbonate diols and triols and higher functionality polyols in relatively small amounts to provide improved impact resistance in acrylic melamine coatings.
There is a need in the art for coating compositions with improved impact resistance. Attempts have been made to produce tougher, more chip-resistant coatings, particularly for automobiles, but these have not been completely satisfactory. In view of the some of the desirable properties of melamine as a crosslinking agent, it would be particularly desirable if it were possible to obtain acrylic melamine coatings with improved impact resistance, with minimal effect on other properties.
In the present invention it has been found that modified acrylic melamine coatings with improved impact resistance can be achieved through incorporation of polytrimethylene carbonate diols and triols, and higher functionality polytrimethylene carbonate polyols. Furthermore, these improvements were observed while maintaining high gloss, weather resistance, and overall durability. It has surprisingly been found that to have both high impact resistance and high gloss, polytrimethylene carbonate diols and triols, and higher functionality polytrimethylene carbonate polyols within a specific molecular weight range provide the best results.
In a related embodiment of the present invention it has also been found that a new baked coating composition can be prepared without acrylic from poly(trimethylenecarbonate), optionally substituted with a glycol, and a melamine/urea formaldehye which provides a number of formulating options for coatings manufacturers. Various formulations have been demonstrated to have desirable levels of adhesion, mar resistance, and low haze.
In accordance with the foregoing, the present invention in the first embodiment comprises: A modified acrylic coating composition cured with melamine characterized by improved impact resistance which comprises:
a) An acrylic polyol dissolved in a suitable solvent to 40-90% solids;
b) 2 to 50% by weight of said acrylic polyol substituted with a polytrimethylene carbonate polyol selected from a polytrimethylene carbonate diol, a polytrimethylene carbonate triol, or a higher functionality polytrimethylene carbonate polyol;
c) A melamine crosslinking agent;
d) Optionally a catalyst; and
e) Optionally pigments and other additives known in the art and used in coatings.
In the second embodiment, where no acrylic is incorporated, the invention comprises:
A melamine/urea formaldehyde polytrimethylene carbonate coating composition comprising:
a) 5 to 80% by weight polytrimethylene carbonate, optionally blended with 0 to 30% glycol;
b) 5 to 70% by weight melamine crosslinking agent;
c) 0 to 70% solvent; and
d) Optionally a catalyst
The compositions can be applied over a variety of substrates.